Diamond is considered a good material candidate for power electronics with the capability of providing power switching efficiency, reliability, and small system volume and low weight. However, single-crystalline diamond (SCD) is ultra-stable and chemically inert to most reactive reagents due to the strong σ-bonds formed between its adjacent carbon atoms. As a result, substitutional doping of single-crystalline diamond is very difficult.
Ion implantation has been attempted to achieve substitutional doping of SCD. However, the ion implantation process needs to be carried out at elevated temperatures to prevent bulk phase transition-graphitization. In addition, a very high post-implantation temperature anneal under a high vacuum is required to restore the damaged lattice and to activate implanted dopants. During this annealing process, and for high dose implantation in particular, surface graphitization still occurs, thereby creating additional unwanted processing complications for practical applications.
An alternative approach to ion implantation is in-situ doping during the epitaxial growth of diamond. However in-situ doping has a number of intrinsic limitations for practical use (e.g., selective doping) due to the need for high temperature and high density plasma during growth. For example, realizing uniform doping concentrations across a diamond substrate using plasma enhanced epitaxial growth is rather challenging due to high plasma concentrations near the edges of diamond substrates. The small size of diamond as a substrate worsens the non-uniformity doping problem of in-situ doping. Using in-situ doping also negatively affects film and crystal quality during plasma assisted epitaxial growth.
In a previous study, a solid boron film was deposited on polycrystalline diamond in order to allow boron doping. While a very high temperature was needed in this process and the feasibility of this method in the context of SCD doping is unknown, diodes made of polycrystalline diamond are undesirable because they show very high leakage current.